Particulate silica referred to as fumed silica that is produced by a dry method is known to have an effect of imparting high viscosity and thixotropic nature to liquid when added to the liquid. With such a characteristic, it is widely used as a thickener for water, liquid resins, paint or the like.
Meanwhile, when the above particulate silica is used as a thickener, it is desired from the viewpoints of the production cost and workability of a product to be obtained that its amount to be added be reduced. Particulate silica exerting a high thickening effect even when added in a small amount is demanded.
To meet the demand, various proposals have been made to improve the thickening effect of the above particulate silica.
For example, there has been proposed a method for producing particulate silica by thermally decomposing a volatile silicon compound in flame, wherein siloxane is used as the raw material volatile silicon compound to improve the thickening effect of the particulate silica to be obtained (refer to EP38900 and EP92024).
Further, in the above patent literatures, it is also described that halogenated silicon such as silicon tetrachloride (hereinafter also referred to as “tetrachlorosilane”) may be used in combination with siloxane.
Although the above particulate silica exhibits a high thickening effect when added to a liquid resin or the like, a thickening effect to be obtained may be varied significantly by the capability of agitator. The thickening effect cannot be obtained by gentle agitation. Further, to obtain the thickening effect by gentle agitation, the amount of the particulate silica to be added must be increased.
It is assumed that the above phenomenon occurs because the method described in the above patent literatures primarily use a silicon compound such as siloxane as a raw material in thermal decomposition in flame. That is, since siloxane has an effect of increasing flame length as compared with silicon tetrachloride which is generally used in production of particulate silica, the increased flame length intensifies fusion of primary particles of particulate silica, and as aggregation caused by the fusion proceeds, massive aggregates increase. Thus, when shear force of agitation or the like is low, aggregates resulting from strong fusion are not crumbled easily, and the particulate silica cannot exert a sufficient thickening effect and are unable to exert a stable thickening effect easily.